The invention relates generally to the field of strip line directional couplers, and in particular to a technique for adjusting the coupling factor of a strip line coupler with at least two strip lines disposed on a circuit board.
Strip line couplers are typically constructed of several strip lines, which for example are arranged opposite one another on a circuit board. For example, the strip lines can be strip conductors applied to the circuit board by printed circuit technology.
Strip line couplers are used to transfer HF signals from one strip to another by coupling. Consequently, they are used in multiple branch circuits for HF signals (e.g., in community antenna systems to distribute the signals received to several users). To assure a perfect signal line and coupling, a precisely defined coupling factor must meet prescribed tolerances. However, this can be achieved only with difficulty, because the thickness of the circuit board and the dimensions (e.g., the thickness, width and spacings) of the copper conductor strips vary considerably. Consequently, the coupling factors vary greatly and as a result a relatively large number of strip line couplers are often discarded during production as rejects.
Therefore, there is a need for a technique for precisely adjusting the coupling factor of strip line couplers.
Briefly, according to the present invention, a strip line directional coupler includes a controlled amount of potting compound applied to establish a desired coupling factor between at least two cooperating strip lines.
In one embodiment the strip line directional coupler is positioned in a housing that is filled with the potting compound until the desired coupling factor is established. A network analyzer monitors the coupling factor as the potting compound is added. Once the desired coupling factor is established, the flow of potting compound into the housing is automatically terminated.
Potting compound changes the electrical properties of the strip line coupler, and the invention uses this to change the electrical properties of the strip line coupler in a specific way.